The present invention relates to an automatic fall back and restore system for data communication and, more particularly, to an automatic fall back and restore system for automatically falling back and restoring a transmission speed in data communication using a data modem having a receiver with an automatic equalizer for equalizing communication line distortion.
In a conventional system, when transmission line degradation occurs and a large number of transmission errors inhibit normal data transmission during data communication between two data transmission apparatuses, a fall back or drop back scheme is employed to reduce the transmission speed to an extent that minimum system transmission can be assured. An error margin for the causes of transmission line degradation is increased to assure normal data transmission although the transmission speed is decreased. A conventional data transmission system using a fall back scheme is disclosed in U.S. Pat. No. 4,438,511. In this data transmission system, the transmission speed is not greatly (e.g., 1/2) decreased at the time of transmission line degradation, but the number of carriers is decreased to achieve fine adjustment. However, the restore mode is not described in this prior art.
The fallen back transmission system is restored (i.e., the transmission speed prior to the transmission degradation is restored) when the transmission line degradation is restored upon a lapse of a given time. In this state, normal data transmission is resumed.
According to the automatic fall back scheme, a predetermined discrimination level corresponding to a critical value of normal data transmission is set in a data transmission apparatus. When a reception signal reaches the discrimination level, the transmission speed is automatically reduced. Whether the reception signal exceeds the discrimination level due to transmission line degradation can be easily determined by the degree of spreading of a reception signal eye.
In the restore mode, the transmission speed must be actually restored to the speed prior to degradation and restoration of normal operation cannot be assured even if a predetermined period of time has elapsed after the transmission system had fallen back. This is because the transmission speed is reduced and the error margin is increased due to the fall back state and it is difficult to discriminate without actual restore operation whether normal transmission can be performed by restoring the system having operated at the reduced transmission speed. It is impossible to perform automatic restore under such an indefinite condition.